Many articles of manufacture are formed by stamping sheet metal blanks between opposing, complementary, unheated forming dies carried in a vertical stamping press. In making such articles, the manufacturer considers the shape to be formed, selects a suitable sheet metal alloy and its metallurgical microstructure, obtains stamping blanks of the sheet material, and stamps the parts by closure of the stamping dies on each sheet metal blank or other workpiece. Such stamping practices have long been used in high volume operations to form automotive vehicle body panels and the like. There is a need to reduce the weight of vehicle parts and a concomitant need to form body structures of streamlined shapes, and of complex shapes with deep pockets, sharp angles and other complex three-dimensional configurations. The sheet metal materials used have varied from relatively low-strength, mild steel to higher strength-to-weight materials, such as high-strength steel, aluminum alloys and magnesium alloys.
There is interest in broadening the range of aluminum alloys used in vehicle bodies, particularly higher strength aluminum alloys, to enable yet further mass reduction. Many of these higher strength aluminum alloy compositions tend to harden, even at ambient temperatures, by a process called age-hardening, after they have been prepared in heat-treated sheet form for stamping. The selected aluminum-based alloy composition is cast into a suitable slab and reduced to a specified sheet thickness (often about 0.5 to 5 millimeters) by a sequence of hot rolling and cold rolling steps. The strained sheet material, whether in the form of a coil or of blanks cut for stamping, may be heat treated at the mill, prior to shipping to a user, to soften it for subsequent shaping in a stamping press. But many desirable aluminum-based alloys, such as the 6000 series of commercial aluminum alloys containing, for example, small amounts of magnesium and silicon, and the 7000 series of aluminum alloys containing, for example, copper, magnesium, and zinc, tend to age-harden. The coiled or blanked material slowly hardens at temperatures experienced during varying shipping and storing periods, into metallurgical microstructures from which the less-formable sheet metal cannot be readily stamped into many of the complex three-dimensional shapes wanted for vehicle applications and other articles of manufacture.
There is, thus, a continuing need for improved methods and processes for forming complex parts from such age-hardenable, high strength aluminum sheet alloys.